Solar cells have been manufactured using a light absorption layer formed at high cost and silicon (Si) as a semiconductor material since an early stage of development. To more economically manufacture industrially applicable solar cells, structures of thin film solar cells, using an inexpensive light absorbing material such as copper indium gallium (di)selenide (CIGS) or Cu(In, Ga)(S, Se)2, have been developed. Such CIGS-based solar cells typically include a rear electrode layer, an n-type junction part, and a p-type light absorption layer. Solar cells including such CIGS layers have a power conversion efficiency of greater than 19%. However, in spite of potential for CIGS-based thin film solar cells, costs and insufficient supply of indium (In) are main obstacles to wide applicability and availability of thin film solar cells using CIGS-based light absorption layers. Thus, there is an urgent need to develop solar cells using In-free or In-less low-cost universal elements.
Accordingly, as an alternative to the CIGS-based light absorption layer, CZTS(Cu2ZnSn(S,Se)4-based solar cells including copper (Cu), zinc (Zn), tin (Sn), sulfur (S), or selenium (Se), which is an extremely cheap element, have recently received much attention. Advantageously, CZTS has a direct band gap of about 1.0 eV to about 1.5 eV and an absorption coefficient of 104 cm−1 or more, reserves thereof are relatively high, and CZTS uses Sn and Zn, which are inexpensive.
CZTS hetero-junction PV batteries were first reported in 1996, but CZTS-based solar cells are technologically less advanced than CIGS-based solar cells and photoelectric efficiency of CZTS-based solar cells is 10% or less which is much lower than that of CIGS-based solar cells. Thin films of CZTS are prepared by sputtering, hybrid sputtering, pulsed laser deposition, spray pyrolysis, electro-deposition/thermal sulfurization, e-beam processing, Cu/Zn/Sn/thermal sulfurization, and a sol-gel method.
Meanwhile, PCT/US/2010-035792 discloses formation of a thin film through heat-treatment of a substrate using ink including CZTS/Se nanoparticles. Generally, when a CZTS thin film is formed with CZTS/Se nanoparticles, it is difficult to enlarge crystal size in the subsequent process of forming a thin film due to previously formed small crystals. As such, when each grain is small, interfaces are extended, causing electron loss at interfaces. Accordingly, efficiency is inevitably deteriorated.
Accordingly, nanoparticles used in a thin film should include Cu, Zn and Sn, and should not be a CZTS crystal type. However, disadvantageously, metal nanoparticles composed of a single metal element may be easily oxidized and require a subsequent additional process for removing oxide at high temperature using a large amount of Se. In addition, when chalcogenides including respective metals are synthesized respectively and used in combination in the process of preparing ink, or used in combination with metal nanoparticles, a non-uniform metal composition ratio may raise a problem and insufficient intermixing between metal nanoparticles having different phases may lead to formation of secondary phases in thin films.
Therefore, there is an increasing need to develop a technology for thin film solar cells including highly efficient light absorption layers that are stable against oxidation, include sufficient amounts of Group VI elements, have an overall more homogenous composition, minimize formation of secondary phases in thin films and have increased film density.